1. Field of the Invention
This invention relates generally to screw anchors for fastening fixtures and other objects against the outer surface of a hollow wall, and more particularly to a removable and reusable plastic screw anchor formed by a four-legged shank which normally assumes a diamond-like shape and is secured by resilient webs to a socket, the shank being collapsible to form a tongue that is insertable in a hole drilled in the wall to accommodate the socket into whose bore a fixture-holding screw is ,inserted which acts to transform the diamond-like shank behind the wall into a triangular truss.
2. Status of Prior Art
In hollow walls formed of plaster board, "Sheetrock," plywood and other relatively thin panel material, one is often faced with the problem of mounting on the wall drapery transverse rods, lighting fixtures and other objects which impose a heavy stress on the wall. Such walls are usually incapable of retaining standard screws, and it is for this reason that an anchor of some sort is required in order to secure a screw to the wall to afford a strong and reliable fixture installation.
The nature of the anchor must be such that it is adapted to engage the inner surface of the hollow wall so that the anchor cannot be pulled out and will not twist when the screw is inserted and taken up to secure the fixture to the wall.
One commonly used screw anchor makes use of a metal sleeve which is longitudinally split to create parallel ribs extending between a flanged front socket received in a hole drilled in the wall and a rear socket, the rear socket being threaded. The arrangement is such that when a screw is inserted to engage the rear socket, turning of the screw causes the ribs to deform and to flare out until they are drawn flush with the interior surface of the wall. The flating of the ribs spreads the load over a relatively wide area of the wall adjacent the hole drilled therein.
One major objection to a metal screw anchor of this type is that there is no efficient way of removing the anchor from the wall; for once the ribs are deformed to their flared condition, they cannot be straightened. Hence to remove the screw anchor, one must break the flange off the front socket and push the remainder of the screw anchor into the wall. This not only destroys the screw anchor so that it cannot be reused, but damage is often done to the wall in an attempt to remove the flange.
To overcome the drawbacks of conventional screw anchors, the patent to Kapnek, U.S. Pat. No. 3,487,746 discloses an anchor formed by parallel ribs which are pivotally secured to front and rear sockets, a spring causing the ribs to pivot and to be held tight against the interior of the wall. The anchor can be straightened out by the insertion of a screw to expand the spring, thereby permitting the withdrawal of the anchor from a hole made in the wall. While an anchor of this type is removable and reusable, its relatively complex structure makes the manufacture and assembly thereof expensive.
Dwyer, in his U.S. Pat. No. 3,213,746, discloses a less costly screw anchor which is molded of plastic material. The anchor includes a flexible web member with a wish-bone-like shape which, when a screw is inserted, spreads apart to hold the socket of the anchor in the wall. In this arrangement, the inner surface of the wall is engaged only by teeth formed on the sloped surfaces of the webs in a small area surrounding the hole, so that the load is concentrated in this area rather than over broad area required to distribute the load. Such load distribution is essential with relatively weak plaster board and the like; for otherwise a heavy load imposed on the anchor will pull the anchor out of the hole.
Still another approach is that disclosed in the McSherry U.S. Pat. No. 3,651,734 in which a screw socket received in the hole in the wall is coupled to a toggle lock. When collapsed, this toggle is insertable through the hole. When expanded to an over-center stable position, the toggle acts to lock the anchor within the hole. One serious drawback of this anchor arrangement is that once installed it cannot be easily removed.
Of greatest prior art interest is my U.S. Pat. No. 4,197,781 (1980) entitled "Screw Anchor." This patent discloses a removable and reusable plastic screw anchor for fastening fixtures and other objects to a hollow wall. The anchor includes a four-legged diamond-shaped shank whose front legs are joined together by a front apex hinge and whose rear legs are joined to the front legs by side apex hinges, the ends of the rear legs defining a rear apex. Secured by a pair of normally outstretched resilient webs to the rear legs of the shank is a flanged socket whose bore lies in registration with an opening formed in the front apex hinge. The diamond-shaped shank is collapsible to form a tongue that is insertable in a hole drilled in the wall to accommodate the socket, the webs then reverting to their outstretched condition to cause the shank which is now behind the wall to recover its diamond formation.
When a self-tapping screw is turned into the socket bore, it acts to thread the bore. As the screw proceeds beyond the bore to engage the tips at the rear legs of the shank, it acts to spread apart these rear legs and compel the shank to assume the configuration of a triangular truss. The bottom chord of the truss is defined by the rear legs and is pressed against the inner surface of the wall, the truss having a central strut formed by the screw when it extends between the front apex and the socket. To remove the anchor from the hollow wall, one simply withdraws the screw to permit collapse of the truss.
In the screw anchor disclosed in my prior patent, the ends of the rear legs which form the rear apex of the normally diamond-like shank are not interhinged but are disconnected. The tips of the rear legs are engaged by the point of the advancing screw which should then act to spread the legs to transform the diamond-shaped shank into triangular form. But in practice, this necessary transformation in certain circumstances does not take place at all or otherwise results in only a partial triangulation of the shank.
These circumstances sometimes arise when the installer drills a hole in plasterboard or other hollow wall material which is relatively fragile. Unless the drilling action is carried out with reasonable care, the plaster around the edge of the hole at the inner surface of the wall will disintegrate or shatter. As a result, a raised annular ridge or bump of plaster particles is developed around the hole edge. When the screw anchor is later inserted in this hole, the annular ridge surrounding the hole edge then may abut the resilient webs which join the socket to the rear legs of the shank and can act to resist spreading of the rear legs by the advancing screw and retard proper triangulation of the shank. Since the ridge is not uniform in shape but is usually irregular, the degree of resistance it offers and how it is distributed varies from hole to hole.
Because of the resistance to triangulation of the shank offered by the ridge surrounding the rear en of the drilled hole in the wall, three possible situations may arise with my prior patented screw anchor in which the ends of the rear legs terminate in tapered tips which define the rear apex of the shank. These tips are quite flexible and easily deflected; hence when the rear legs of the shank whose tips are supposed to cause triangulation meet resistance because of the ridge, the tips deflect, and in doing so fail to carry out their required function.
In the first possible situation in which the rear legs encounter resistance to normal displacement, when the fixture-holding screw is inserted in the socket bore and turned, the screw then first advances toward the flexible tips at the rear legs of the shank. These tips, whose axial displacement is resisted by the ridge then lie between the crests of the screw and engage the root thereof. The root diameter of the screw is substantially smaller than that of the crests; hence the extent to which the screw is then capable of forcing apart the rear legs and overcoming the resistance of the ridge is limited by the diameter of the root. As a result, the shank is only partially triangulated and does not provide a triangular truss whose bottom chord is pressed against the inner surface of the wall to attain maximum anchor holding power.
Moreover, in order for the screw to be effective in the first situation, its length must be sufficient to bridge the distance between the inner surface of the wall and the front apex of the partially triangulated shank to form the center shank of the truss. This screw length must be greater than the length that would be required had full triangulation occurred; for then the front apex of the shank would be brought closer to the rear surface of the wall.
Apart from the fact that a longer and hence a more expensive screw is required, is the fact that in many instances the depth of the hollow portion of the wall is insufficient to accommodate a longer screw.
A second situation that is sometimes encountered is one in which as the screw is advanced, because of the resistance offered by the ridge to normal triangulation, the tips of the rear legs are laterally deflected by the advancing screw and the entire screw then spirals between the rear legs, instead of pushing the rear legs axially forward. As a consequence, the shank behind the wall remains in its diamond-like shape and not even partial triangulation takes place, so that even when the screw is fully inserted in the socket, the installed anchor does not attain its full holding power.
Though in the second situation no triangular truss is formed to provide full holding power, the installer who cannot see behind the wall is then unaware that the installation is defective.
The third situation represents a worst case possibility; for instead of the tapered tips falling between the screw crests as in the first situation or being laterally deflected so that their sides lie flat against the screw crests as in the second situation, what now happens is that because of uneven resistance offered by the ridge around the drilled hole, one rear leg is engaged by the point of the screw and acts to triangulate this leg, while the other tip is laterally deflected and lies parallel against the screw. As a result, one rear leg of the shank is somewhat displaced by its tip which later engages the root of the screw, whereas the other rear leg whose tip is laterally deflected, is not displaced and the shank is then caused to assume a distorted diamond-like shape.
Because of the distorted diamond, the hole in the front apex is no longer in registration with the socket bore and the formation of a truss is no longer possible, for the tip of the advancing screw will now strike one of the front legs and will miss the front apex hole. Here again, though the installation is defective, the installer who cannot see behind the wall is unaware of this fact.
Under normal conditions, the rear legs of the triangulated shank form the chord of a triangular truss which is pressed along its entire length against the inner surface of the wall, so that the load is distributed over a broad area. But under those circumstances where a raised ridge surrounds the edge of the drilled hole at the inner surface of the wall, the center portion of the chord of the truss will be pressed against this ridge and the end portions of the truss which are farthest from the hole will then be disengaged from the wall; hence the load will be concentrated in the region surrounding the hole.
Because of the inherent weakness of plasterboard, this load concentration is undesirable; for with a heavy load, the anchor may rupture the plaster board in the hole region and be pulled out of the wall by the load. Thus even when proper triangulation has occurred with my prior patented anchor, its full holding power is not attained.
This does not mean that my prior screw anchor is ineffective--for tens of millions have already been put to successful use--but only to the extent there is a build-up of plaster particles or other material in the region surrounding the edge of the hole--and this depends on the care exercised by the installer and the quality of the drill used to form the hole--this build-up militates against the proper function of the screw anchor.